During an offset printing process paper travels through multiple printing units which sequentially apply different image separations or components to the paper. This process requires the application of ink and fountain solution to the paper, which often changes the dimensions of the paper. One factor contributing to such growth is exposure of the paper to liquids. As the paper receives each application of ink and fountain solution, the paper expands resulting in an overall increase in the dimension perpendicular to the grain of the paper. In a web offset press, the grain of the paper usually runs parallel to the length of the paper web, and hence, growth typically occurs across the width of the paper web. Although the web width continually expands as the web passes through each printing unit, each successive increase is smaller than the previous increase, resulting in differential growth.
Because growth alters the original dimensions of the paper, the printed color images do not align correctly and create distorted images. This misalignment of the color images is referred to as misregistration. During a four-color printing process, the colors cyan, magenta, yellow and black are typically used to build a spectrum of colors. For example, to produce green, the colors cyan and yellow are overlapped. When paper growth occurs, the printed colors become misaligned, leaving a blue edge on one side of the image and a yellow edge on the other side of the image. These misregistered colors undesirably reduce image reproduction quality. To correct such gross misregistration of images, printing presses must frequently be stopped during the printing process and realigned to reduce waste. This method of correction is costly and has resulted in significant press downtime.
Historically, various techniques have been developed to adjust for registration variances caused by paper growth. Two well-known methods of correcting registration errors are the shift method and the trap method. The shift method makes adjustments to the alignment of individual page color separations in prepress by shifting the page image for each separation into a position that will cause the images to register in the center of each page when paper growth occurs. One disadvantage to this method is that shifting often brings page images into close, but not perfect, register. The trap method requires printing ink over previously printed ink to abut misaligned color images. Like the shift method, the trap method corrects for misregistration by making misalignment less noticeable, yet does not completely eliminate registration errors. Also, trapping is a complicated process utilizing algorithms that require large amounts of CPU processing time when performed by a computer. Trapping algorithms can further result in undesirable image appearance in certain circumstances. In addition, trapping can result in increased ink usage, in turn leading to the possibility of substrate over-saturation and resultant smearing and offsetting which contribute to undesirable quality and detract from the reasons for trapping in the first place.
Other methods track the movement of a paper web through the printing press by using detecting means, usually an optical monitoring assembly, to obtain data for correction of misregistration. In U.S. Pat. No. 5,365,847, a control system for a printing press is described in which a pair of sensing assemblies located on opposite sides of the web detect and gather information concerning fan-out or expansion of the paper web. Within the control system, a central processing unit receives signals from the sensors and automatically corrects the misaligned printing image by supplying the appropriate signals to electronic image devices associated with the printing cylinders of the press.
In U.S. Pat. No. 5,806,430, a digital printing press includes an automatic register adjustment for a plurality of digital imaging units to correct register errors occurring during the printing process. Specifically, the press includes an error detection device having sensors which detect printed registration marks to generate register error signals, an image modification circuit connected to the sensors for receiving the error signals to modify image data and thereby correct register errors and a raster image processor to receive the modified image data in preparation for printing. Register adjustment can be carried out continuously during the printing process.
While the above-mentioned solutions correct misregistration by employing the use of measuring instruments and image processing equipment to make adjustments to the images during the printing process, these techniques are not desirable because of the high costs of the necessary equipment to implement same. Furthermore, current methods to correct misregistration errors are objectionable because they only correct errors occurring during the printing process.